1. Field of the Invention
The present invention relates to a direct conversion tuner that includes two mixers and that outputs an I signal and a Q signal having orthogonal phases.
2. Description of the Related Art
A conventional direct conversion tuner will be described with reference to FIG. 7. For example, a digitally modulated high-frequency signal is received by a receiving unit 41 including a bandpass filter 42 and a low-noise amplifier 43 provided at a next stage, and is then distributed into two by a distributing circuit 44. The distributed high-frequency signals are input to a first mixer 45 and a second mixer 46. The first mixer 45 and the second mixer 46 receive local oscillation signals from an oscillation circuit 47. The first mixer 45 receives a local oscillation signal directly, while the second mixer 46 receives a local oscillation signal via a 90° phase shifter 48. Thus, the local oscillation signals supplied to the two mixers 45 and 46 have a phase difference of 90°. The local oscillation frequency coincides with, for example, the frequency of the high-frequency signal received.
Thus, the two mixers 45 and 46 output baseband signals, and an I signal output from the first mixer 45 and a Q signal output from the second mixer 46 have a phase difference of 90°. In order to avoid image interference, these baseband signals are output to subsequent stages via bandpass filters 49 and 50, respectively. This scheme is described, for example, in Japanese Unexamined Patent Application Publication No. 11-331011 (FIG. 11).
The baseband signals having passed through the bandpass filters 49 and 50 are added together by an adder circuit that is not shown, and then the resulting signal is demodulated.
When television signals in the UHF band and the VHF band are to be received using the direct conversion tuner described above, since the frequencies of these bands are separated from each other, two independent oscillators respectively for the UHF band and the VHF band are needed in an oscillation circuit. Furthermore, two PLL circuits for controlling oscillation frequencies, associated with the respective oscillators, are needed. Therefore, the overall construction becomes complex.